The present invention relates generally to signal communication involving spread spectrum signals. The present invention is particularly useful for and applicable to signal-discriminating receiver arrangements involving direct-sequence spread spectrum signals.
Spread spectrum systems involve the communication of twice modulated signals. The first modulation is applied to a carrier signal that is used to carry information, and the second modulation is used to gain certain advantages in transmission. These advantages include, among others, coding the transmitted signal for information privacy, and minimizing the likelihood of the signals interfering with or jamming the transmitted spread spectrum signal. The second modulation purposefully xe2x80x9cspreadsxe2x80x9d the signal in the frequency spectrum. One type of spread spectrum signal, referred to as a direct-sequence spread spectrum (xe2x80x9cDSSSxe2x80x9d) signal, can be created by spreading the spectrum of a modulated carrier signal by directly modulating the modulated carrier signal using a wideband spreading waveform. For a system to be classified as a spread spectrum system, typically the signal energy transmitted by the system occupies a bandwidth larger than and independent of the information bit rate. In most applications, the bandwidth is much larger than the information bit rate.
The transmitted signal is demodulated in a spread spectrum receiver using a correlation process in which the received signal is correlated with a replica of the signal used in the transmitter to spread the signal through the bandwidth. For spreading DSSS signals, a pseudorandom noise (PN) binary sequence is typically used. Despreading DSSS signals involves proper synchronization of the spreading waveform, and the receiver accomplishes this using a replica of this same PN binary sequence. In some implementations, the output of a PN generator in the receiver is multiplied with the incoming bit stream. The resulting signal is filtered using a bandpass filter centered at the carrier frequency, and then processed using a detector that determines if there is a match. If a match is not present, the process is repeated with altered timing in the PN sequence. Once a match is discovered, the receiver switches from this coarse signal acquisition mode to a tracking mode in which the receiver attempts to maintain phase lock for proper alignment between the received PN sequence and the generated replica PN sequence. The duration of the bit pulse in the waveform used to spread the signal in the second modulation is referred to as the xe2x80x9cchip intervalxe2x80x9d (or sometimes xe2x80x9cchipxe2x80x9d), and its inverse is referred to as the xe2x80x9cchip rate.xe2x80x9d
The receiver""s course signal acquisition mode and subsequent tracking mode are, of course, a critical part of any DSSS receiver. Assuming that coarse acquisition has been carried out to within an accuracy to one half chip using one of many conventional techniques, useful information is available only after accurate recovery using the tracking mode. For this reason, achieving rapid tracking acquisition is highly desirable. For the sake of brevity, unless otherwise specified the term xe2x80x9cacquisitionxe2x80x9d hereinafter refers to fine acquisition in the tracking mode.
In packet-type spread communication systems, for example, rapid acquisition reduces the required length of a preamble, which in turn increases net data throughput. Acquisition time can also be traded for receiver power consumption in ranging applications such as communication systems that include position-estimation data as a part of the communication process. Such systems include, among others, cellular phones adapted to comply with the Federal Communication Commission""s enhanced xe2x80x9c911xe2x80x9d mandate, personal locator systems for the elderly, personal navigation systems, the Global Position System, the Russian Global Navigation System, and CDMA systems generally. To accommodate wide use and application of such systems, the receivers employed for analyzing the communicated position-estimation data should be sufficiently small and power efficient to satisfy conventional portability demands.
For many of these applications and according to aspects of the present invention, it would be advantageous to reduce power consumption by occasionally disabling the receiver""s signal processing circuitry. Once the signal processing circuitry is disabled, however, reactivating the signal processing circuitry requires reacquisition, coarse and fine, of the received signal. Thus, achieving rapid tracking acquisition is highly desirable because it permits the signal processing circuitry to be more frequently disabled and, due to a shorter time needed to retrack, disabled for longer periods of time.
Tracking, or merely xe2x80x9cacquisitionxe2x80x9d in this context, is generally achieved using a delay-locked loop (xe2x80x9cDLLxe2x80x9d) with a fixed loop gain. The time required for acquisition is approximately proportional to the loop gain, and the DLL""s steady state variance is inversely proportional to the loop gain. Thus, any fixed choice of a loop gain represents a trade-off. To achieve both requirements, time-varying loop gains in the context of digital phase-locked loops (DPLL) have been proposed. For various reasons, however, these proposals have not been fully embraced for many applications requiring fast-tracking.
Various implementations of the present invention are directed to the receiver end of a communication system involving a fast-tracking discrimination approach useful for rapid synchronized acquisition of direct-sequence spread spectrum signals. Some of these implementations are specifically directed to communication systems involving code-division multiple access (CDMA) and to the types of communication systems discussed above.
In connection with the present invention, it has been discovered that by selecting a set of consecutive correlation results from several timing offsets of a direct-sequence spread spectrum signal, an absolute timing error can be obtained from the assumption that there is a polynomial relationship among the consecutive correlation results. One implementation of the present invention is directed to a receiver having a DLL employing discrimination that is approximately linear and independent of the received signal power.
With reference to certain example method and apparatus embodiments, the present invention involves a receiver arrangement having a reference clock for receiving a signal over a delay path, the signal being modulated by a waveform having a bit rate defined in terms of a bit-pulse duration (chip). The signal is rapidly acquired by: for at least one reference code, obtaining correlation results from at least several timing offsets of the signal, wherein each timing offset is separated by a portion of a chip (e.g., about one third); selecting at least three consecutive correlation results; and providing the selected consecutive correlation results as inputs to a polynomial relationship having a degree of at least two, and determining therefrom a timing error useful in receiving the signal.
In another particular embodiment, the above-characterized approach uses a quadratic relationship. In a more specific example embodiment, for at least one reference code, correlation results are obtained from several timing offsets of the signal, where each timing offset is separated by about one third of a chip. Using five consecutive ones of these correlation results, assuming that the correlation energy of band-limited signals is approximately quadratic, an accurate phase error is obtained for synchronizing the received signal. When this embodiment is used in an optimal variable gain filter, the receiver outperforms the normalized classical early-late discriminators despite the unrealistic assumption that such classical discriminators have an exact priori knowledge of the received signal power.
Other aspects of the present invention are directed to particular methods, arrangements and systems involving the above-characterizations. For instance, example applications include systems adapted to synchronize data relatively quickly and/or adapted to process position-estimation data (e.g., based on time of flight or multiple-point angle measurements).
The above summary is not intended to characterize every aspect, or each embodiment, contemplated in connection with the present invention. Other aspects and embodiments will become apparent from the discussion in connection with the FIGS.